1. Field of the Invention
The present invention relates to an optical readhead for reflectance devices, and more particularly, to an accurate means of measuring diffuse, spectral reflectance over a wide variation in the distance between the sample and the illuminating source and detector
2. Description of the Prior Art
Analytical chemistry as a field of technology in connection with testing and determination of components in the presence of substances has rapidly been advanced in recent years, especially with regard to the specialized area of biochemistry which has emerged as a primary scientific frontier in modern times. There exists a need for increasingly sophisticated analytical methods and devices to make determinations involving new techniques which were never attempted heretofore. The explosion in the medical research field has encouraged the growth of the field of analytical chemistry with an emphasis on high precision, speed and simplicity in obtaining reproducible results. The growth and development of other industries such as brewing, chemical manufacturing and other have also resulted in rapid strides being made in the field of analytical chemistry.
A variety of analytical procedures, compositions and apparatus have been developed in order to meet the needs of these expanding fields and technologies including solution chemistry techniques, automated machinery and the so-called "dip and read" type reagent strips.
Because of their relatively low cost, ease of use and speed of obtaining results, reagent strip test devices enjoy wide popularity in many analytical applications, especially in the field of testing of biological fluids. In medicine and health areas, for example, numerous physiological functions and problems can be monitored and checked merely by dipping reagent test strips into a sample of body fluids such as urine and blood. By observing a detectable response, such as a change in color or a change in the amount reflected from or absorbed by these test devices, a particular health condition or body disfunction can be quickly determined even by persons who are not highly trained or skilled in the chemistry or medicinal technologies underlying these conditions. Many of these test devices produce a semi-quantitative detectable response and many efforts have been made to refine these devices to the point where almost quantitative results are obtained. For example, by measuring the response after a predetermined time, the analyst or observer can obtain not only a positive indication of the presence of a particular constituent or component in a test sample, but also an estimate of how much of the constituent is present, at least within parameters that provide the scientist or physician with an initial approximation of the problem or condition. Such test devices provide the physician with a facile diagnostic tool as well as the ability to rapidly and quickly gauge the extent of disease or body malfunction.
Many such test devices are currently available and are on the market including those available from the Ames Division of Miles Laboratories, Inc. under the proprietary designations CLINISTIX.RTM., MULTISTIX.RTM., DIASTIX.RTM., DEXTROSTIX.RTM. and others.
Typically, test devices of the so-called dip and read type include one or more carrier matrices, such as bibulous paper or other absorbent material, having incorporated therein a particular reagent or reactive system which manifests a color change in the presence of a specific sample component. Depending on the reactant system incorporated into a particular matrix, these devices can detect the presence of glucose, ketones, bilirubin, urobilinogen, occult blood, nitrite and other substances. The specific time range after contacting the test device with sample is indicative of the presence of a particular component and the concentration in the sample. Some of these test devices and their reactant systems are set forth in U.S. Pat. Nos. 3,123,443; 3,212,855; 3,184,668; 3,164,534; 2,981,606; 3,298,789; 3,164,534; 3,092,465 and 2,981,606.
Automated instruments have been developed over a period of years which are intended to eliminate the need for manual manipulation of chemical reactants and which have programmed readouts which greatly facilitate and speed up such tests. Such devices are intended to improve reproducibility of tests by eliminating the subjectivity of the operator as a factor in obtaining reliable measurements. The emphasis on obtaining reliability and reproducibility of test results in today's climate of concern over accuracy and precision in laboratory test procedures is well known. Various reflectance instruments have been employed which use a light source and a photoelectric cell or other sensing means to determine color values by measuring the amount of light reflected from a colored surface illuminated by the reference light source.
One factor in being able to obtain reliable and accurate readings is the matter of positioning of the light source and the detector means in such a way as to minimize the effects of specular (that is mirror-like) reflections that occur at the surface of a test device being measured.
When dealing with reflectance measurement from surfaces of diagnostic reagent strip test devices, it is particularly important to avoid specular reflections and to obtain the measurement of nonspecular reflected light in the reflectance apparatus. Information on analyte concentration as a function of color development of a reagent test surface is contained only in the diffusely reflected energy. The specular component therefore represents a source of "noise" or inaccuracy in the reflectance measurement. As the state of the art moves toward more quantitative results, the ability to resolve small differences in reflectance--and hence small changes in analyte concentration--will in part be limited by the instrument's ability to suppress such "noise".
Some earlier efforts have been made to minimize the adverse affects of specular reflectance by changing the axis of detection relative to the axis of illumination. This is shown, for example, in U.S. Pat. Nos. 3,604,815 and 3,907,503. Other devices have proposed the use of multiple light sources positioned at an angle of 45.degree. to the normal and collecting the reflected light normal to the specimen as shown in U.S. Pat. No. 4,279,514. The geometries involving a 45.degree. angle have been considered most important in making reflection measurements as reported by the International Commission on Illumination publication CIE No. 44(TC2.3) 1979 entitled "Absolute Methods for Reflectance Measurements".
For the purpose of reducing or eliminating the adverse affects of specular reflection, previously described apparatus has been constructed which incorporates a diffusing chamber or integrating sphere; see for example U.S. Pat. No. 4,171,909. These designs do provide diffuse light to illuminate a specimen, however, the incorporation of an integrating sphere into such designs automatically increases the size and expense of the required optical readhead.
Textiles have often been the subject of reflectance measurement in order to analyze the surface thereof to determine certain properties of the textile. One such apparatus is shown in U.S. Pat. No. 4,033,698 which utilizes fiber optic bundles for transmittal of light to and from the textile sample area. This design appears to be quite costly and technically sophisticated However, the design does not appear to effectively minimize stray light rays originating from the light source and thus the adverse affects of specular reflectance.
Another optical system for analyzing the surface of a fibrous web is shown in U.S. Pat. No. 4,490,618 which analyzes the surface of paper or textile using a prism structure with one surface of the prism in contact with the fibrous web under a predetermined pressure. A collimated beam of light is directed into the prism on the contact surface. The light reflected from the contact surface to the prism is then directed to a detector.
Still a further example of use of a collimating lens to make the light rays substantially parallel for the purpose of testing the surface of a composition is shown in U.S. Pat. No. 3,776,642 where a testing device is used to determine the nature and characteristics of the surface of a quantity of grain to be analyzed.
In addition to the issue of included specular reflections, it is well known that the accuracy and precision of making diffuse reflectance measurements is highly dependent upon the distance from the test sample to the light source and the light detector. For example, if the sample moves farther away from the light source and detector, the magnitude of energy reaching the detector is decreased. To the instrument, this change in energy level is indistinguishable from an equivalent change due to color development of the sample.
This invention concerns the problem introduced by the translucency of a sample. The term "translucency" or "translucent" is defined as the property of transmitting some portion of the incident radiation.